Methods for the manufacture of capacitors are varied depending upon the nature of the capacitor and the energy storage requirements. In electronics, low dissipation factor and small size are primary requirements. In other applications the size of the energy storage device is less important than cost. In yet other applications, rapid delivery of the energy stored in the capacitor is a paramount concern.
In the field of energy storage, capacitors are generally recognized as advantageous. In the past, pure electrostatic capacitors have usually been the least energy dense and one of the most expensive devices to store bulk energy. Despite their limitations, electrostatic capacitors have found widespread use in electronics due to their ability to deliver very high power rates. This very attractive feature is due to the ways in which the power is stored within the capacitor. For example, since the discharge of a capacitor does not generally depend upon the movement of electrochemical species in a relatively macro environment, the power delivered by a capacitor is generally at least several orders of magnitude greater than a similarly sized electrochemical battery.
Capacitors are also generally able to withstand relatively low temperatures and relatively high temperatures. Many types of capacitors perform in temperature ranges of −30° C. to 120° C. Extension of these ranges with controlled or linear capacitances is also a desirable feature.
Unfortunately, capacitors are also generally characterized by high cost per unit energy stored per volume or weight. Use of electrostatic capacitors for bulk energy storage has been severely hampered by the high unit costs in this application. A reduction in the unit cost per unit energy stored is desperately needed by the world's increasing needs for energy storage.
By way of background, assuming a 1 cubic meter volume and using units of the mks system, it can be shown that energy is proportional to permittivity and inversely proportional to the square of the thickness or distance between electrodes, as follows:
  U  =                              e          0                ⁢                  KV          2                            2        ⁢                                  ⁢                  d          2                      =                            e          0                ⁢                  KE          2                    2      where, U=energy                V=Voltage between the electrodes        d=distance between electrodes        K=Relative Permittivity        e0=permittivity of vacuum        E=Electric Field (V/d)        
The thinnest dielectric at the highest voltage possible (largest E-field) will provide the highest energy density possible at a given relative permittivity, K. The highest voltage possible varies greatly depending upon the material used for the dielectric. To obtain the highest energy storage levels, the dielectric should be very nonconductive, have a good permittivity and be as thin as possible.
Any conductivity between the electrodes is termed leakage current and is to be avoided. At some voltage level the dielectric will become conductive, by either the leakage current rising to unacceptable levels or the leakage current rising dramatically in a fraction of a second (usually accompanied by a plasma spark). The limit of the E-field value varies greatly depending upon the molecular chemical nature of the dielectric and the morphology of the dielectric material.
As a general rule the more polar a molecule in the dielectric, the higher the dielectric constant (i.e., relative permittivity). And, as a general rule the high dielectric breakdown voltage materials tend to have low permittivity. Exceptions to those general rules are certain compounds, such as barium titanate or other Perovskite types of mixed metal oxides (ceramics). Those types of compounds we can see both high permittivity and good resistance to dielectric voltage breakdown. However, another problem then occurs when these types of dielectrics are pushed to energy storage levels that are beyond their capabilities. In particular, metal oxide ceramics have difficulty maintaining high permittivity at large E-fields (voltages). As an example, it has often been found that the permittivity of barium titanate at high E-fields results in an over 100 times reduction in permittivity versus the low E-field permittivity. Thus, the need for a high E-field breakdown material with simultaneous high permittivity is needed in electrostatic capacitor devices. It is therefore important that the voltage rating for the capacitor be as high as possible when energy storage is the primary use for the device.
In addition to having a high break down voltage, a high energy density capacitor should also possess an extremely low leakage current. Thus, when the capacitor has been charged to a given voltage, the rate of charge conduction from one electrode to the other should be a relatively small value. When the capacitor is charged for energy storage over a given period of time, the rate of leakage is an acceptably low enough value that would vary depending on the use of the storage device (how long is it stored) and the “value” of the energy thus stored (how easy is it to recharge and the cost of the charge). While an acceptable value for leakage may vary greatly from application to application, leakage is undesirable and to be avoided and minimized.
Heretofore it has been recognized that the addition of insulative materials to the dielectric matrix can cause an unwanted diminution in the value of the dielectric breakdown strength. In general this is true. Also the construction of a capacitor is governed by the geometric construction of the device. A multilayer dielectric is generally not preferred for a film capacitor. Setting aside the complications involved in forming several layers between the electrodes for the dielectric, the overall gain of energy storage is usually little if any. This is caused by the reduction in the E-field that is necessary when the layers are diminished in thickness.
Due to the desirable characteristics of electrostatic capacitors and other undesirable features, an improvement in the methods and materials for the construction of these energy storage device and improved capacitors incorporating these materials are needed. The invention is directed to overcoming one or more of the problems and solving one or more of the needs as set forth above.